The isolated perfused rat liver system (IPL) was used to study the kinetics of uptake and excretion for four polychlorinated biphenyl isomers (PCBs). All four were rapidly taken up from the perfusate by the liver, and the rate of biliary excretion was greatest with the least chlorinated isomer (4-chlorobiphenyl) and slowest with the most chlorinated isomer (2,4,5,2',4',5'-hexachlorobiphenyl). The rate of biliary excretion for the two less chlorinated isomers was exponentially related to their hepatic concentration, but for the two more highly chlorinated isomers excretion was not related to hepatic concentration.